1. Field of the Invention
The invention relates to the preparation of magnetic bubble device material, and in particular to the use of ion implantation for hard bubble suppression.
2. Description of the Prior Art
The use of ion implantation of gases such as helium and neon into the layer of a magnetic bubble device material for hard bubble suppression is known in the prior art.
Hard bubbles are those which collapse at bias fields up to 50 percent higher than normal bubbles, move at an angle to the magnetic field gradient used for propagation, and move at a lower velocity than normal bubbles. Hard bubbles must be suppressed in all circuit applications as a potential source of data error. The complex wall states responsible for the hard bubble behavior are inhibited from forming if a domain wall covers an end of the cylindrical bubble domains. In the prior art, ion implantation of Noble gases has been the most convenient and wide-spread means for suppressing hard bubbles.
However, there are several disadvantages with the use of these gases, such as the relatively long amount of time required to perform the ion implantation, and the use of non-standard implant material in the layer.
In ion implanted films, when the stress-induced anisotropy of the ion-damaged region exceeds the K.sub.u of the bubble material, a layer of in-plane magnetization forms which is separated by a 90-degree domain wall from the underlying bubble material. Obviously, small bubble compositions with higher K.sub.u will require higher implantation-induced anisotropy to suppress hard bubbles. Since K.sub.u increases rapidly as the temperature is reduced, an investigation of hard bubble suppression over the temperature range was carried out on small bubble material using both ion implantation and an alternate technique.
During ion implantation, gaseous ions which have been accelerated by a high electric field strike the garnet surface and penetrate some distance, causing atomic displacements that expand the garnet lattice. The implanted region is constrained from expanding parallel to the film surface by the relatively great thickness of substrate crystal below, which places the implanted layer in compressive stress. Acting through the inverse magnetostriction effect, this stress produces an induced anisotropy, K.sub.i, given by EQU K.sub.i =-(3/2).sigma..lambda..sub.111,
where .sigma. is the stress and .lambda..sub.111 is the magnetostriction constant of the bubble material. If .lambda..sub.111 is negative, the compressive stress causes K.sub.i to be negative. (By convention, negative K.sub.i creates an easy axis parallel to the film plane). If K.sub.i exceeds the K.sub.u of the as-grown bubble material, an implanted in-plane magnetization layer forms. When this implanted layer is sufficiently thick, a 90-degree domain wall is formed between the in-plane magnetization and the perpendicular magnetization of the underlying undamaged bubble material.